Refrigerated Storage Volume using Air Conditioner

ABSTRACT

A conversion unit for an air conditioning system to cause it to act like the cooling unit of a refrigeration system comprises a frost detector, an A/C temperature control defeating mechanism and a control unit which operate together to force the range of operation of the air conditioning unit into the range of operation of a refrigeration unit. The conversion unit is particularly useful for providing low cost cooling systems for farmers in third world countries for keeping their produce fresh and safe, not to mention its use by all farmers around the world and by florists or others in need or desire of an economical refrigeration alternative. The present invention is also usable to provide inexpensive cooling to RV&#39;s and to refrigerated vehicles.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of copending application Ser. No.12/803,540, filed Jun. 29, 2010, entitled “Retrofittable air conditionerto refrigeration conversion unit”, which was a continuation-in-part ofapplication Ser. No. 11/724,129 filed on Mar. 14, 2007. Theaforementioned applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally directed to the conversion ofconventional air conditioning system units so as to operate more capablyas part of a complete refrigeration system. Thus, as compared toconventional air conditioning units, the present invention permits theeconomical construction of insulated volumes which are now capable ofbeing cooled to near freezing levels. More particularly, the presentinvention is directed to a separate unit, which is configured with aconventional air-conditioning unit in order to convert it to the core ofa refrigeration system, as opposed to its function as a mere airconditioner.

2. Description of Related Art

It is well known that air conditioning units are relatively inexpensive.They can often be purchased for amounts even as low as $30 in somedeveloping countries; domestic prices can be found as being as low asaround $150. In contrast, refrigeration systems that are employed incommercial settings tend to be expensive and have relatively high powerdemands and installation requirements. Accordingly, it is seen thatthere is a need for a mechanism, which is capable of converting aninexpensive air-conditioning unit so that it operates as the core of arefrigeration system.

However, it is not well known that air conditioning units are typicallydesigned to shut down their compressor operations when it gets too cold.For the typical room size air conditioner, this temperature is around64.degree. F. but may be as low as 60.degree. F. For example,Frigidaire® has made an air conditioner unit in which a temperaturesensor is disposed in thermal contact with the coils of the device. Whenthe coils become “too” cold, the compressor is shut down. One of thereasons that they do this is to control the viscosity of the refrigerantsince efficiency is adversely affected by having the refrigerant passthrough an expansion valve (orifice) which is too small relative to thefluid's viscosity. Later models of the Frigidaire® units eliminate thistemperature sensor and instead rely solely on the room air temperaturesensor which is still used to shut the compressor down at a temperatureof around 60.degree. F.

It is also noted that not all air conditioning units employ a thermistortype of temperature sensor. Again, this temperature sensor is for roomair and is intended to determine the point at which the unit shuts downto avoid further cooling. Some A/C units, such as those typified byunits installed in recreational vehicles, employ a long copper capillarytube as part of an analog design. These units typically employ anintegral switch assembly with a rotatable analog setting knob whichcontrols the lower cutoff temperature based on temperatures to which therelatively lengthy capillary tube is exposed. Such units are found inrecreational vehicles (RVs) and in older air conditioners as well as ininexpensive air conditioners sold in developing countries. The presentinvention includes an embodiment that is compatible with this A/C designas well and allows RV air conditioners to operate large truck coolingsystems at a cost of around $2,400 instead of the present cost of around$14,000.

It is further noted that there is a significant need for inexpensiverefrigeration systems. In particular, farmers would very much like tohave an inexpensive method for keeping their produce and crops atreduced temperatures were for storage and for longer shelf life.Additionally individuals such as florists, restaurants, and grocerystores would also benefit from having inexpensive refrigeration systems.Furthermore, as desirable as these systems are in the United States,they are immeasurably more desirable in other parts of the world whererefrigeration is at a premium but which is nonetheless a necessitybecause of the elevated temperatures of the climates in these regions.

In addition to the fact that refrigeration systems are expensive, it isalso the case that such systems are very demanding in terms of theirelectrical power requirements. It is therefore seen that there is also aneed for cooling systems that require less electricity than is consumedby conventional cooling systems which the current invention incombination with a standard window air conditioning unit is capable ofdoing.

One of the problems with using a conventional air-conditioning unit aspart of a refrigeration system is that such units are designed withspecific controlling features in mind, which limit their operations,cycle duration and their cooling capabilities. For example, the controlunits for a conventional window air conditioner are set so that theunits turn off at a relatively high sensed temperature. Nonetheless, forpurposes of using a conventional air-conditioning unit as the core of arefrigeration system, these air conditioners, with their conventionalcontrol units, are set up so that it is always far from the case thathumidity is allowed to condense on the fins of the unit in the form ofice. In short, in their normal mode of operation, conventionalair-conditioning systems are designed to cut out at a relatively hightemperature. It is therefore seen that in their off-the-shelf state,these units are not capable of operating as refrigeration units. Theadapter units of the present invention provide a retrofit mechanismwhich extends the range of operation for a conventional air conditioningunit. This is found to be particularly advantageous in relatively smalland inexpensive window units.

SUMMARY OF THE INVENTION

Accordingly, in order to solve these problems, there is provided asimple retrofittable conversion unit, which includes a frost detector, acontrol unit and a heater, which is used to “fool” the temperaturesensor in a conventional air conditioner. The present inventioncomprises a device to adapt an air-conditioning unit to a lowertemperature of operation. The device comprises a sensor for detectingthe presence of frost on the fins of the air conditioning unit and aheater for disposition adjacent to a temperature sensor for the airconditioning unit. A control unit deactivates the heater upon thecondition that the sensor provides an indication that there is frost onthe fins. There is also provided a method of installation of the presentdevice so that it easily works with a conventional, off-the-shelf A/Cunit.

In accordance with another embodiment of the present invention, there isprovided a corresponding method for operating an existing airconditioning unit having fins across which air is directed to cool it,so as to achieve a lower temperature of operation. This method includesthe following steps: applying heat to a temperature sensor present inthe air conditioning unit; sensing the presence of frost on the fins theair conditioning unit; and controllably adjusting heat applied to thetemperature sensor to produce continued operation without producingsignificant frost build up on the fins.

In accordance with yet another embodiment of the present invention,there is provided a method for the conversion of an existing airconditioning unit into a unit capable of operating as the core of arefrigeration system which operates at near freezing temperatures. Inthis method a heater in an adapter is thermally connected to thetemperature sensor of the air conditioner. A frost sensor in the adapterunit is disposed adjacent to the fins of the air conditioner. Theadapter is electrically connected to the air conditioner to supply theadapter with power. These steps may be performed in any convenientorder.

In accordance with yet another embodiment of the present invention, theheater is replaced by a relay which replaces the integralknob/capillary-tube switch described above. Instead of driving anelectric heater, the present invention also encompasses the concept ofentirely replacing the temperature control of an A/C unit with a relay.This is easy to accomplish since the integral knob-capillary tubeswitches are typically provided as a single pluggable (and thereforeunpluggable) unit.

In yet another embodiment of the present invention the heater isreplaced by wiring directly into the air conditioner control board inplace of and at the point of the air conditioner's temperature sensorprobe and then providing synthesized variable resistance values thatmimic colder or warmer readings from the original air conditioner'stemperature sensor.

The present invention is also characterizable as an auxiliary controlmechanism for an air conditioning device that already has a controlmechanism albeit one that precludes its operation below a certaintemperature.

Accordingly, it is an object of the present invention to bring theadvantages of refrigeration to areas of the country and the world whereit is most needed and least affordable.

It is also an object of the present invention to provide a retrofitmechanism which extends the range of operation of conventional A/Cunits.

It is a still further object of the present invention to provide amethod for easy installation of the present device.

It is yet another object of the present invention to provide aneffective and economical refrigeration system.

It is also an object of the present invention to provide an economicaldevice and system for the improved preservation of produce together withall of the health and food safety benefits that that entails.

It is a still further object of the present invention to provideinexpensive A/C cooling systems for use in refrigerated trucks and/orother commercial vehicles.

Lastly, but not limited hereto, it is an object of the present inventionto provide an add-on device for controlling A/C units so as to make themusable in conjunction with easily implementable insulated or insulatablevolumes which can be kept at near freezing temperatures.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention.

The recitation herein of a list of desirable objects which are met byvarious embodiments of the present invention is not meant to imply orsuggest that any or all of these objects are present as essentialfeatures, either individually or collectively, in the most generalembodiment of the present invention or in any of its more specificembodiments.

BRIEF DESCRIPTION OF THE DRAWING

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the concluding portion of thespecification. The invention, however, both as to organization andmethod of practice, together with the further objects and advantagesthereof, may best be understood by reference to the followingdescription taken in connection with the accompanying drawings in which:

FIG. 1 is a front view of a conventional room or window air-conditioningunit, illustrating the typical controls provided with such a unit;

FIG. 2 is a block diagram view of a conventional room or windowair-conditioning system;

FIG. 3 is a block diagram of the retrofit apparatus of the presentinvention used to modify the conventional operation of room or windowair-conditioning units in order to provide a refrigeration function;

FIG. 4 is a front view of a human interface panel that is employablewith the present invention;

FIG. 5 is a block diagram illustrating the control circuit and theoverall structure of the present invention;

FIG. 6 is a diagram illustrating the adapter of the present inventionbeing employed with a surrounding insulated structure through whose backwall a conventional air conditioning unit is disposed;

FIG. 7 illustrates the use of the present invention as a replacementsystem for a large refrigerated truck;

FIG. 8 is similar to FIG. 3 except that a relay is employed instead of aheater; and

FIG. 9 is also similar to FIG. 3 except that a variable resistancetemperature signal generator is employed instead of a heater.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a conventional room or window air-conditioning unit100. The front of such units typically include vent openings 102,through which cooled air is supplied to a room. Such units alsotypically include vent openings 104, through which room air may beexhausted. In particular fans or other air moving devices are operatedin reverse mode under control of switch 107. Switch 107 is typicallyprovided with the control function of supplying air to the room orremoving air from the room in an exhaust mode. Such a mode of operationis conveniently provided so that the fan provided with unit 100 iscapable of supplying cooler evening air from the outside through vents102 while at the same time, exhausting warmer interior air throughexhaust vent 104. Switch 107 controls this function. Additionally thereis also provided temperature control switch 106, which allows a user tochoose a temperature below which the unit ceases its cooling function.Once a desired temperature is reached, the unit's compressor function isshut down. However, the units fan may continue operation for apredetermined time following the determination that he desired roomtemperature has been reached. Conventional unit 100 also includescontrol switch 105, which selects the mode of operation. In one mode ofoperation, a user may select to operate only the fan and not the unit'scompressor or cooling function. This may be desirable for example, insituations in which a simple exchange of room air with outside air isdesired. Mode control switch 105 also typically provides to other modesof operation: hi cool and low cool. The hi cool mode of operation is onein which greater electrical current is supplied to either or both of thefan motor and or compressor motor to select either the degree and orspeed of cooling.

Since the structure and operation of the present invention is based upona modification of the conventional system employed in off-the-shelf roomand window air conditioners, it is appropriate to consider the usualrefrigeration cycle and the controls that are normally imposed thereon.Accordingly, the structural block diagram shown in FIG. 2 is provided inorder to enhance one's understanding of the parameters and controlsinvolved. In particular, it is seen that motor 200 drives compressor202, which compresses a refrigerant. This refrigerant flows throughconduit 203 to expansion valve 204. In expanding through this valve, thecompressed refrigerant is cooled in accordance with well-knownthermodynamic principles. The cooled fluid is passed through conduit 205to condenser 206. Condenser 206 includes fins across which fan 208 blowsair which is cooled via its thermal contact with the fins of condenser206. In condenser 206 refrigerant is warmed by the passage of air acrossits fins and the fins are cooled by being in thermal contact with therefrigerant which has been cooled by its passage through expansion valve204. Thus warmed coolant is returned via conduit 207 to compressor 202at which point the cycle repeats.

Motor control 210 controls the operation of compressor motor 200, andfan motor 201. Under control of selector switch 107 fan 208 may beoperated in reverse to provide an exhaust function. More particularly,motor control 210 responds to signals input from temperature sensor 209.Motor control 210 also receives input signals from switches 105, 106 and107 shown in FIG. 1.

The present invention provides a retrofit apparatus, which is used tobetter control the conventional refrigeration cycle illustrated in FIG.2. Since the normal temperature range of operation for a room or windowair conditioner is not so low as to cause ice buildup on the fins ofcondenser 206, there is no need in such units to provide for frost orice detection. Since these units have not been contemplated for use asthe central core of a refrigeration system, as opposed to a simple roomair cooling system, frost or ice detection has not been seen as either adesired or necessary function. However, if one wished to use such unitsin any refrigeration function where the temperature range issignificantly lower, frost or ice accumulation is a problem.Accordingly, one of the elements provided in the retrofit apparatus ofthe present invention is frost or ice sensor 400 as shown in FIG. 3.This is preferably implemented as temperature sensor, however, anyconvenient means for detecting frost may also be employed includingelectrical conduction and/or optical sensors.

Additionally, as noted above, conventional room or window airconditioners are not designed to function below certain temperatures.Such units are designed essentially for cooling a room not for turningit into a refrigeration structure. Accordingly, the retrofit apparatusof the present invention also includes heater 500, which is disposed inclose proximity to temperature sensor 209. Control 300 operates toactivate heater 500 so as to effectively fool temperature sensor 209.However, it is noted that by choosing to operate at lower temperatures,frost or ice detector 400 is employed, whereas before no such sensor wasneeded or desired.

Accordingly, it is seen that the present invention provides a retrofitapparatus having three complements. Heater 500 is employed toessentially force the air conditioning unit to operate so as to producelower temperature air. Frost or ice sensor 400 is employed to ensurecontinued operations at the lower desired temperature, which is more inthe range of a refrigeration system than in the range of a room coolingsystem. Control unit 300 separately receives a user supplied indicationof desired temperature. Using heater 500 and sensor 400, control unit300 operates to control the conventional room or window air-conditioningunit in the manner described above. In preferred embodiments of thepresent invention, heater 500, sensor 400 and control unit 300 areprovided in a single package, which is easily connected into and coupledwith a conventional room or window air-conditioning unit to provide arefrigeration function.

FIG. 5 is a block diagram illustrating the various components of theadapter of the present invention. In particular microcontroller 300 isimplementable as PIC Microchip microcontroller Model No. 16F916, thoughmany low-end microcontroller chips would also be just as satisfactory.This chip contains code burnt into an EEPROM for implementing thecontrol algorithm and user interface functions described above. There isincluded also included internal heater 510 included on the main circuitboard to prevent short-circuits due to condensation. Internal heater 510is controlled by controller 300. As described above, there is alsoprovided external heater 500 connected via wires 450. This heater isdisposed as described elsewhere herein. The heater itself is located onan external cable that plugs into the main circuit board. Heater 500 isalso controlled with controller 300. Also provided is power on indicator426 which is lit when DC power is connected. Two temperature sensors 330and 335, measure the room temperature and the temperature of the airconditioner's fins respectively. These sensors are mounted on externalcables that plug into the main circuit board. Controller 300communicates with them using a serial protocol to read the twotemperatures at appropriate times. Three input buttons, 410, 415 and 420are accessible from the front panel and are used to change theparameters of the cooling algorithm, as well as for diagnostic purposes.Their functions are also described in greater detail elsewhere herein.Display 405 is made up of two modules, DIS1 and DIS2. The display isprovided in the present implementation solely as a matter ofconvenience. The relevant aspect of the display is that there are asufficient number of digits to display the temperature or any optionaldiagnostic settings. Controller 300 uses these digits to display runningstatus, to provide feedback while the user sets algorithm parameters,and to support diagnostic tests. There are also preferably two statusindicator lights (470 and 480 in FIGS. 4 and 5). Indicator 470, which iscontrolled by controller 300, is lit when the control algorithmdetermines that the air conditioner should be turned on. Indicator 480,which is also controlled by controller 300, is lit by when the controlalgorithm determines that current is required in heater 500 in order toheat it to a level that will trigger the air conditioner to turn on.

Attention is now directed to a method by which the present invention isadded to an existing air conditioning unit. The first step in thisprocess is the construction of an insulated volume. Materials useful inthis process include Styrofoam and SprayFoam which can be applied toseal any cracks or gaps in the structure. At this stage, one should alsoconsider adding extra insulation. If there are windows present in thestructure, they should be sealed with Styrofoam or any other useful oravailable insulative material.

If it does not already exist, a conventional air-conditioning unit isdisposed through an opening in the structure wall. The edges of theopening are sealed as well. The next step is the removal of the frontportion of the air-conditioning unit. This front portion is typicallyplastic. Its removal also typically exposes air filters present in theunit. These air filters are also preferably removed. It is recommendedthat this front portion not be reinstalled. This exposes the fins of theair conditioning unit which produces both an advantage and adisadvantage. The disadvantage is that the fins can be bumped and bent.The advantage is that the fins can easily be cleaned and be bent backinto shape as needed.

The next step in the installation procedure is the location and thefreeing of the thermocouple sensor that normally comes with the airconditioning unit. Note that this freeing operation is not an electricaldisconnection, but rather a moving of the thermocouple away from thefins of the air conditioning unit. Typically the thermocouple isdisposed on a long and flexible wire, which is easily bent away from thefins. If there are any plastic ties or other structures holding thethermocouple in place, these are preferably removed as well so as tohave the thermocouple swing free of the fins.

The next step in the installation process is the mounting of the deviceof the present invention on a wall of the structure near the airconditioning unit. Here on this device is referred to herein as theConversion Unit, The Conversion Unit may be provided with any convenientwall fastening means, including screws, adhesives, Velcro or even hungon nails. The Conversion Unit is hung on the wall in a positionsufficiently close to the air conditioning unit that wires extendingfrom the Conversion Unit are capable of being connected to appropriatepoints on the air conditioning unit.

The next step in the installation process is the mating of thethermocouple with the warming element of the Conversion Unit. Thiscoupling is designed to ensure close thermal contact between the twoelements. In particular, it is possible to join these two elements byplacing them next to one another and wrapping them with aluminum foil.Even a single layer of the aluminum foil is adequate; however, multiplelayers provide a more secure coupling.

The next step in the installation process is the connection of theConversion Unit's frost sensor to the fins of the air conditioning unit.Looking at the fins in a typical air-conditioning unit, one sees thatthere are copper pipes carrying the unit's refrigerant. The frost sensoris disposed, just below one of the lower copper pipes, which istypically several inches above the bottom of the air conditioning unit.The frost sensor is inserted between two of the fins. One may rely upona friction fit to hold a frost sensor in place or more preferably, onemay bend some of the adjacent the fins together to more ably hold thefrost sensor in position. This is easily done with one's fingernails orwith a screwdriver

For air-conditioning units, which are Energy Star compliant, there is anadditional step that is also performed as part of the installationprocedure. In particular, the frost sensor that normally accompaniessuch units is moved. Note, however, that this sensor is not removed onlyrepositioned away from the fins so that it does not interfere with theoperation of the Conversion Unit. The Conversion Unit is also providedwith an ambient room temperature sensor. This sensor should be allowedto hang freely in the cooled volume. The present invention thus rendersit very easy to retrofit a conventional room or window air-conditioningunit so as to operate as the core of a refrigeration system. The onlyother thing that needs to be provided is some form of insulated airtightstructure. Wood and Styrofoam structures, which are readily available inrural and third world areas readily suffice for carrying out thisfunction.

Attention is now directed to a view of the front panel of ConversionUnit device 400. In particular, the front panel includes LED (or othertechnology) display 405 which is used to not only display the currenttemperature, but is also used to set desirable temperatures to beachieved at the air conditioner fins. It is noted that any convenientdisplay device may be employed and the display is not limited to LEDdevices; LCD displays are employable; however, it is noted that in manyrefrigeration environments lighting may be so low that LED displays area significantly preferred choice. Likewise, LED displays are preferredin situations where condensation may be a factor. Front panel 400includes the three buttons labeled room, frost, and delay (havingreference numerals 410, 415, and 420, respectively).

Pressing the “ROOM” Button lets one pick the desired room temperature.In current preferred embodiments, the lowest selectable temperature is32.degree. F. Every time you push the ROOM button, the temperature goesup one degree; when it reaches a maximum temperature, it then startsover again at 32.degree. F. Pressing the “FROST” button allows one tochange the frost detection settings. The Conversion Unit device isprovided with a default temperature setting for this value, but if iceforms on the fins, pressing the FROST button so that it goes up one ortwo degrees typically stops this from happening. If the room isn'tgetting cold and ice is never forming, then the frost temperature is settoo high. Pressing the FROST button until it cycles back to the startingpoint should solve this problem. If no frost is ever forming theneither: (1) one has a new Energy Star compliant unit and didn't move itsfrost sensor; (2) the room is extremely leaky and uninsulated; or (3)the room is too big for the given air conditioning unit. The solution tothese last two problems is sealing the room better, adding a second A/Cunit or using a single larger unit.

Pressing the DELAY button changes the Delay Mode. This button controlsthe delay between the time that both sensors' temperatures are abovetheir respective thresholds and when the air conditioner is triggered tooperate. The default is ten seconds. Increasing the delay allows theroom to get warmer before triggering the air conditioner. This issometimes useful for air conditioners that have an enforced minimumon-time due to internal control circuitry, so that they run for theirminimum on-time without frost forming before they may be turned off.

The Conversion Unit shown in FIG. 4 also includes several wires ordevices, which need to be connected to the air-conditioning unit. Inparticular, lead 425 is connected to a DC power source. While aconventional battery could be employed to power the electroniccomponents in control unit 400, the demands of heating element 500 whichis placed in thermal contact with thermocouple 209 means that it issignificantly more preferred to connect unit 400 to a separate DC powersupply. In preferred embodiments of the present invention, the DC powersupply is from a converter, which is coupled into the alternatingcurrent power supply for the air-conditioning unit. It may also bepowered separately. Also shown in FIG. 4 is lead 435 which is connectedto frost sensor 400. Likewise, lead 430 is connected to heater 500.These items are considered in the discussion above with respect to FIG.3.

FIG. 6 illustrates a typical installation of the adapter described abovein its natural environment. This drawing is not to scale, so that all ofthe features and aspects may better be presented. In particular,conventional but modified air conditioner 100 is disposed through a backwall of insulated structure 600. Air conditioner 100 is depicted as ifit had its front cover removed. Adapter device 400 is shown connected toair conditioner 100 in three ways: (1) via DC electrical connection 425;(2) via frost sensor 335 (not visible) connected via wire 435; and (3)via heater 500 connected to temperature sensor 209 via connection 450.FIG. 6 also illustrates the presence of alternating current power outlet620 into which a power cord from air conditioner 100 is inserted (notshown for reasons of improving clarity of the view). Also shown in FIG.6 is room temperature sensor 330 connected to unit 400 via connectingcable (wire) 430.

Attention is now directed to a description of specific control methodsemployed in the operation of the Conversion Unit device. For purposes ofefficiency, it is desirable to turn the air conditioner unit on and offwith as little lag time as possible, that is, with the shortest possibledelay between when the algorithm says “A/C off” and when the A/C stopsemitting cold air. Turn-on delay is primarily limited by how fast thesensor heater warms up; turn-off delay is limited by how fast it coolsdown. To be more precise, turn-on delay is the time it takes for theheater to go from its “turned off” temperature past the airconditioner's threshold temperature, and turn-off delay is the time ittakes for the heater to go from its “steady state on” temperature pastthe air conditioner's threshold in the other direction. The turn-ondelay is smallest when the “turned off” temperature is high (but belowunder the air conditioner's threshold temperature) and when the currentput through the external heater 500 is largest. The turn-off delay issmallest when the “steady state on” temperature is low (but greater thanthe air conditioner threshold temperature) and when the room temperatureis much cooler than that temperature. Accordingly, desirable performancelevels are achieved if the current supplied to heater 500 is justsufficient to keep the temperature just above the air conditioner'sthreshold temperature while it's “on” and to supply slightly lesscurrent while it's “off.”

One way to accomplish this control is through the use of a PID(proportional-integral-derivative) control. This allows for precisecontrol of the heater's temperature, but it also requires an additionalsensor for feedback, which adds an expense, both for parts and forassembly. However, in certain cases where precise control over a longperiod of time is desirable for reasons of efficiency, this expense istolerable.

The following items further describe a control algorithm and method forusing the Conversion Unit device: When turning on the heater, keep itcompletely on (full current flow) for an initial length of time,approximately one minute; however, this value may be increased ordecreased depending on the current room temperature. Use a relativelylow resistance for heater 500, so that it “sees” a relatively highcurrent and gets hot fast. After the initial turn-on period, use PulseWidth Modulation (PWM) to reduce the average current through the heater,and thus the heat dissipated. This stabilizes the heater temperature,rather than driving it as hot as it can possibly go. Thus, when theheater is turned on, its lower temperature results in faster turn-off

In short, PID control is an option but there is a trade-off between costand performance. Using PWM to reduce steady-state temperature makes forfaster turn-off and customizing the PWM period (100% initially,decreasing later) makes for faster turn-on.

The use of PWM as a form of control is also relevant to a cooperativecontrol method for supplying power to internal heater 510 and externalheater 500. Internal heater 510 is used to reduce condensation on thecircuit board. Its use is also beneficial in that it contributes to theelimination of a manufacturing step in which a conformal coating isapplied to the circuit board and its components to ameliorate problemsassociated with condensation and/or other environmental contaminants.However, it is noted that, if internal heater 510 runs constantly, peakcurrent load goes over one ampere in the current design. A DC powersupply capable of producing more than one ampere often costssignificantly more than one designed for less than one ampere.Accordingly, control in the Conversion Unit device turns internal heater510 off whenever external heater 500 is on. Since only one heater is onat any given time, the peak load is not the combined load but themaximum. It's much easier to keep this under one ampere and to thusemploy a less expensive power supply.

As indicated above, certain air conditioners do not employ a thermistorfor temperature sensing purposes. Instead such vehicles typically employthe above mentioned capillary tube temperature probes. Such probes arenot easily “fooled” by the use of a heater. Therefore, in accordancewith one embodiment of the present invention, the temperature controlunit that is supplied with the air conditioner is replaced with a relay.For example, see relay 800 in FIG. 8 which is the same as FIG. 3 exceptfor the replacement of heater 500 by relay 800. This is easilyaccomplished since such analog temperature control units are typicallyprovided as plug-in assemblies. Thus, in this embodiment, the entiretemperature control unit is replaced by a relay. This requires nomodification of the above described Conversion Unit device other thanremoval of the heater and its replacement by a relay and a change in thefirmware to provide a continuous signal to the relay as opposed to apulsed signal which is preferably supplied in the embodiment using aheater. This really is a simple switch which is controlled by the samevoltage level used to operate the heater. In this manner, the presentinvention is seen to be also applicable to use in recreational vehicles.

In a still further embodiment of the present invention rather thaneither using a heater, or replacing the heater with a relay, theConversion Unit operates to provide the same kind of information that isexpected by the air conditioner. In particular, in certain circumstancesthe air conditioner is controlled by a thermistor which interpretsvariable electrical resistance as indicative of temperature. Thus, in athird embodiment of the present invention, the defeating means foroverriding operation of a temperature sensor used to determine the lowerlimit for air conditioner operation comprises a single line which alsosupplies a variable resistance to mimic lower and higher temperatures.See variable resistance temperature signal generator 900 in FIG. 9 whichis the same as FIG. 3 except for the replacement of heater 500 byvariable resistance temperature signal generator 900. Thus it is seenthat the defeating means may comprise either a heater, a relay, or avariable-resistance temperature signal-generator.

The present invention is also useful in replacing refrigeration systemsfound in the trucking industry. In particular, for a large truck, such arefrigeration system is typically seen to cost approximately $14,000.However, as shown in FIG. 7, truck 700 is cooled using threeconventional air conditioners (100 a, 100 b and 100 c). These unitsreplace conventional refrigeration unit 701 (shown in phantom form).Assuming that a conventional air conditioner costs approximately $500and a Conversion Unit device costs approximately $300, one sees that onecan provide the same level of cooling for a total of $2,400. The numberof Conversion Units employed is selected based upon the vehicle volumethat is to be cooled. This represents a savings per vehicle of over$10,000. However, given the size of the largest of the refrigeratedtrucks, particularly in terms of their height, it is seen that theadvantages of the present invention are often best achieved when thedevices are deployed within recessed areas of the truck ceilings. Such arecessed unit (100 c) is shown in FIG. 7. For smaller vehicles,disposition in recessed areas is typically unnecessary and is typicallyemployed only as called for by truck height limitations.

While the invention has been described in detail herein in accordancewith certain preferred embodiments thereof, many modifications andchanges therein may be effected by those skilled in the art.Accordingly, it is intended by the appended claims to cover all suchmodifications and changes as fall within the true spirit and scope ofthe invention.

Accordingly, it is to be understood that the embodiments of theinvention herein described are merely illustrative of the application ofthe principles of the invention. Reference herein to details of theillustrated embodiments is not intended to limit the scope of theclaims, which themselves recite those features regarded as essential tothe invention.

What is claimed is:
 1. A refrigerated storage volume, comprising: an airconditioning unit having a temperature sensor with a designed lowercutoff temperature controlling the operation of the air conditioner andfins through which air to be cooled is driven; an insulated volume to becooled to a refrigeration temperature below the designed lower cutofftemperature of the air conditioning unit, the air conditioning unitbeing mounted through a wall of the insulated volume with the coolingfins and temperature sensor inside the volume; a conversion unitallowing the air conditioning unit to cool a volume to a refrigerationtemperature below the designed lower cutoff temperature of the airconditioning unit, comprising: a frost sensor mounted upon the fins ofthe air conditioning unit for detecting a temperature at which frost islikely to form on said fins of said air-conditioning unit; a heatermounted in close proximity to the temperature sensor of the airconditioning unit, such that when the heater is operated the temperaturesensor of the air conditioning unit senses a temperature above thedesigned lower cutoff temperature of the air conditioning unit; a roomsensor for sensing ambient temperature in the volume, disposed in thevolume remote from the heater and the frost sensor on the fins of theair conditioning unit; and a control unit having inputs coupled to thefrost sensor and the room sensor and an output coupled to the heater,the control unit activating the heater when the room temperature sensorsenses a temperature in the volume above a desired refrigerationtemperature, and the control unit deactivating the heater when the frostsensor detects a temperature at which frost is likely to form upon thefins of the air conditioning unit.
 2. The refrigerated storage volume ofclaim 1 in which said control unit further comprises an input forsetting desired refrigeration temperature within the storage volume. 3.The refrigerated storage volume of claim 1 in which said control unitfurther comprises an input for setting a threshold temperature at whichfrost is likely to form on said fins.
 4. The refrigerated storage volumeof claim 1 in which said frost sensor is a temperature indicating probe.5. The refrigerated storage volume of claim 1, wherein the wherein thelower cutoff temperature is no less than 60° F.
 6. A conversion unit toadapt an air conditioning unit having a temperature sensor with adesigned lower cutoff temperature controlling the operation of the airconditioner, and cooling fins through which air to be cooled is driven,the air conditioning unit being mounted through a wall of a volume withthe cooling fins and temperature sensor inside the volume, theconversion unit allowing the air conditioning unit to cool the volume toa refrigeration temperature below the designed lower cutoff temperatureof the air conditioning unit, said conversion unit comprising: a frostsensor mounted upon the fins of the air conditioning unit for detectinga temperature of the fins of the air conditioner at which frost islikely to form on said fins of said air-conditioning unit; a heatermounted in close proximity to the temperature sensor of the airconditioning unit, such that when the heater is operated the temperaturesensor of the air conditioning unit senses a temperature above thedesigned lower cutoff temperature of the air conditioning unit; a roomtemperature sensor for sensing ambient temperature in the volume beingcooled, disposed in the volume remote from the heater and the frostsensor on the fins of the air conditioning unit; and a control unithaving inputs coupled to the frost sensor and the room temperaturesensor and an output coupled to the heater, the control unit activatingthe heater when the room temperature sensor senses a temperature in thevolume above a desired temperature, and the control unit deactivatingthe heater when the frost sensor detects a temperature on the fins ofthe air conditioner at which frost is likely to form upon the fins ofthe air conditioning unit.
 7. The conversion unit of claim 6 in whichsaid control unit includes a display for displaying the ambienttemperature from the room temperature sensor.
 8. The conversion unit ofclaim 7 in which said display is selected from the group consisting ofan LCD display and an LED display.
 9. The conversion unit of claim 8 inwhich said control unit includes a means for setting the desiredtemperature.
 10. The conversion unit of claim 6 in which said controlunit includes a means for setting a threshold temperature on the fins ofthe air conditioner at which frost is likely to form upon the fins ofthe air conditioning unit.
 11. The conversion unit of claim 6 in whichthe volume is a thermally insulated enclosure, the conversion unit andthe thermally insulated volume forming a refrigerated storage volume incombination with the air conditioner.
 12. The conversion unit of claim 6in which said air conditioning unit is disposed through a wall in saidvolume with the cooling fins and temperature sensor within the volume.13. The conversion unit of claim 6 in which said frost sensor is atemperature indicating probe.
 14. The conversion unit of claim 6,further comprising a second heater disposed internal to said device forcontrolling condensation therein.
 15. The conversion unit of claim 14 inwhich power is supplied at distinct times to said heater and said secondheater.
 16. The unit of claim 6 wherein the lower cutoff temperature isno less than 60° F.
 17. A conversion unit for adapting a room or windowair conditioning unit of the type that has cooling fins, a thermocouple,and a lower cutoff temperature of no less than 60° F., so as to cool avolume to less than the lower cutoff temperature, the unit comprising:means for setting and displaying a desired temperature that is lowerthan the cutoff temperature of the air conditioner; means for sensingthe temperature of the cooled volume; means for heating the thermocoupleof the air conditioner so as to cause the air conditioner to cool thevolume to less than the lower cutoff temperature; means for detectingthe buildup of frost on the cooling fins; and means for ceasing theheating of the thernmocouple when frost is detected on the cooling fins.